You can say the paragraphs of Shema as loud or quiet as you want, so long as you can hear yourself say the words. (Also, be smart that you’re not so loud as to disturb other people.)
You say the Brachos yourself and then you answer Amen to the Chazzan (whether or not you say Amen to Ohev Amo Yisrael depends on your Minhag). It’s quite possible that if you rely on the Chazzan for Shome’a K’oneh for just those final few words of the Bracha that you were not Yotzei, though P’shat probably is that you still were.